1. Field of the Invention
This invention relates to SRAM cells and more particularly to a process for fabrication thereof.
2. Description of Related Art
U.S. Pat. No. 4,785,341 of Ning et al for "Interconnection of Opposite Conductivity Type Semiconductor Regions" describes a butting contact structure for SRAM circuit connections.
FIGS. 9A-9C are cross sectional views starting with a semiconductor substrate with a buried contact opening formed in a gate oxide layer and illustrating the process flow of a prior art buried contact process.
In FIG. 9A, a P- substrate 90 is covered with a gate oxide 91 with an opening 92 therethrough down to P- doped silicon substrate 90.
In FIG. 9B, the device of FIG. 9A after an N+ region 93 has been formed below opening 92, followed by the deposit of a blanket first polysilicon layer 94 thereon which is doped conventionally.
In FIG. 9C, the device of FIG. 9B has been coated with a mask 95 over first polysilicon layer 94 with an edge thereof located over N+ region 93 and hole 92. The mask 95 is a patterned photoresist layer 95 adapted for use during etching away the first polysilicon layer 94 at the edge of the opening 92. In view of misalignment of the mask 95 with the edge of opening 92, a trench 96 is formed where the bottom contact opening 92 is open and the photoresist mask as did not provide protection, because the polysilicon etching process which should be stopped by the gate oxide 91, instead, because of the misalignment, etches the N+ and P- silicon substrate during the overetching period.